In recent years, a research into an assessment apparatus which calculates an ultrasonic non-linear parameter by using the amplitude of a fundamental frequency and the amplitude of a harmonic frequency of ultrasound penetrating an inspected object by making the ultrasound be incident in the inspected object and assesses a change in physical property of the inspected object by using the calculated ultrasonic non-linear parameter has been in active progress.
However, when the inspected object is heat-treated and the inspected object is thus degraded, precipitates are generated and thereafter, the precipitates are grown and coupled, and dissoluted and the ultrasonic non-linear parameter also increases and decreases according to the nucleation and growth of the precipitates.
Accordingly, it is difficult to measure clearly and accurately a degradation degree of the inspected object by using the assessment apparatus in the related art. That is, when the inspected object is degraded, the ultrasonic non-linear parameter also increases and decreases as the quantity of precipitates increases and decreases and since the assessment apparatuses in the related art can measure the degradation degree of the inspected object based on only the ultrasonic non-linear parameter at a specific time, the degradation degree may be ambiguous and inaccurate.
Meanwhile, a propagation speed of an elastic wave in a solid is determined by physical properties such as an elastic coefficient, a density, and a Poisson's ratio of a propagation medium. Accordingly, the elastic coefficient of the propagation medium is acquired by measuring the propagation speed of the elastic wave to estimate the physical properties of the propagation medium. A method for acquiring the elastic coefficient of the propagation medium includes a linear elastic coefficient measuring method that measures the propagation speed of the ultrasound by using the ultrasound which belongs to the elastic wave to calculate a liner elastic coefficient.
However, there is a disadvantage in that a minute change and degradation of a micro characteristic or an elastic property of the propagation medium cannot be evaluated by the linear elastic coefficient measuring method. As a method for remedying such a disadvantage, a theoretical research into a correlation between the ultrasonic non-linear parameter and the non-linear elastic coefficient is performed.
As a result of the research, a relationship between a 2nd order ultrasonic non-linear parameter and a 2nd order non-linear elastic coefficient is verified. As described above, the correlation between the 2nd order ultrasonic non-linear parameter and the 2nd order non-linear elastic coefficient is researched, but a research into 3rd or high order ultrasonic non-linear parameters and 3rd or high order non-linear elastic coefficients is not made.
As associated prior art, Korean Patent Laid-open Publication No. 10-2012-0031674 (Title of Disclosure: System and Apparatus for Measuring Non-linearity of Ultrasonic Wave, Apr. 4, 2012) is provided.